1. Field of the Invention
The present invention relates to a sheet processing apparatus for an image forming apparatus.
2. Description of the Related Art
Image forming apparatuses, such as copying machines and laser printers, which are equipped with a sheet processing apparatus are known. The sheet processing apparatus receives sheets discharged from the image forming apparatus after image formation, and performs saddle stitching and book binding by subjecting the sheets to operations, such as for example, center folding, binding almost the centers of the sheets, and folding the sheets in two.
An example of a sheet processing apparatus includes a compact, space-saving, and low-cost book binding device. In order to bind a bundle of sheets, the book binding device first conveys sheets, which have been supplied one by one from a main unit of an image forming apparatus, into a storage guide so that the sheets are stored therein in a substantially vertical position.
The sheets are positioned by bringing leading ends of the sheets into contact with a sheet positioning member that is placed at a predetermined binding position, are aligned in the width direction, and are then saddle-stitched at the center by a stapler. Subsequently, the sheets are folded at the center by a thrust plate and folding rollers so as to make a booklet. The booklet formed by a bundle of sheets is discharged onto an output tray from an output port provided on a discharge side of the folding rollers in a manner such that the folded portion of the booklet is at the forefront (as described, for example, in Japanese Patent Laid-Open No. 2002-331730).
Japanese Patent Laid-Open No. H11-78287 discloses another binding device. In this binding device, a sheet larger than a bundle of sheets is folded into a foldout having the same size as that of the bundle of sheets, and the foldout, a cover sheet, and a bundle of sheets corresponding to one booklet are bound together such that the foldout is provided between the cover sheet and the bundle of sheets. That is, a bundle of sheets including a folded sheet is folded and bound to make a booklet.
In the above-described sheet processing apparatus and the image forming apparatus disclosed in Japanese Patent Laid-Open No. 2002-331730, for example, when a bundle of sheets including a folded sheet is processed (e.g., subjected to book binding), it needs to be aligned. If the folding accuracy of the folded sheet is low, the accuracy in aligning the bundle of sheets and the folded sheet may be decreased. That is, the aligning accuracy may decrease depending on the folding accuracy of the folded sheet. That is, in a case in which the folding accuracy of the folded sheet is low, when an aligning member touches a folded portion of the folded sheet, the aligning accuracy of the entire bundle of sheets including the folded sheet may be decreased.
For example, if the bundle of sheets is subjected to book binding while the ends of the sheets are not aligned properly, the quality of a booklet obtained by book binding is lowered.